#20 enable/disable scheduled tasks

[fabrizio giustina]

add an enabled property to BaseTaskConfiguration that
let you quickly disable a single scheduled task.

This can be used in scheduler configuration to
enable/disable single tasks based on deployment
properties

<FixedRateTask>
<Minute>0</Minute>
<Hour>1</Hour>
<SchedulableComponent>
ref:///xxx/yyy
</SchedulableComponent>
<Enabled>
{/deployment/CurrentDeployment$taskEnabled}
</Enabled>
</FixedRateTask>